Frequency comb generation is motivated by applications in sensing, spectroscopy, RF synthesis, clock distribution and general communications. A frequency comb generally represents multiple (more than two) distinct frequency tones and can include hundreds or thousands of frequencies spread over predefined spectral range. Conventional frequency comb generation is accomplished by using the combination of stabilized optical lasers and nonlinear optical process. These solutions are generally constrained with respect to the variability of frequency spacing between generated tones, width of the spectral band covered by such frequency comb, spectral equalization among generated frequency tones and spectral tone purity. Conventional frequency comb generation generally fails because it relies on techniques that cannot address either the bandwidth, variable frequency pitch, spectral purity, spectral equalization or a combination of these factors.
A frequency comb generator should produce multiple frequency tones over wide spectral range, and in spectrally equalized manner. The spectral equalization is of critical importance in sensing, metrology and communication applications that require the power of frequency tones comprising the comb to be maintained at substantially constant level. When the frequency comb is used for providing multiple carriers (channels) at the transmitter node of the communication, each tone power should be maintained at the same level in order to preserve the same transmission reach for all channels. When the frequency comb is used to provide multiple local oscillators at the end receiver node of the transmission link, the performance of each channel requires that frequency tones possess equal power. The most common realization of frequency comb fails to achieve spectral equalization, and particularly so when the bandwidth of the spectral comb exceeds narrow range of approximately 10 nanometers. Of equal importance, the common realization cannot maintain spectral flatness, variability of frequency comb pitch, comb coherency and bandwidth simultaneously. In most common realizations, these parameters are traded in detrimental manner: spectral flatness is commonly achieved only over narrow spectral band range; high coherency is achieved at the expense of fixed, non-selectable frequency pitch; high coherency is achieved at the expense of using high power, frequency fixed laser seeds.
It would be desirable to generate a frequency comb in a manner that its frequency pitch, defined as spacing between the closest frequency tones, can be varied independently from the stabilized laser source used for its generation. Furthermore, it would be useful that frequency comb can be generated over wide spectral area and particularly over the area exceeding the capabilities of conventional radio-frequency generation means. Still further, it would be also desirable that the generated comb has spectral equalization, defined as difference in power among frequency tones comprising the frequency comb, i.e. frequency tones are generated with substantially equal power. Furthermore, it would be useful if spectral purity, also defined as a coherency, of generated frequency tones remains high. Therefore, there is a current need in the field for new means for generation of variable frequency pitch combs that are spectrally equalized over substantially wide spectral band, while retaining high degree of coherency.